CONTRACTION CHARACTERISTICS AND MYOSIN HEAVY-CHAIN COMPOSITION OF RABBIT MASSETER MOTOR UNITS

1. We studied isometric twitch peak force (TPF) and twitch contraction time (TCT) of 249 motor units of the masseter muscle in 41 rabbits after extracellular electrical stimulation of single trigeminal motoneurons in the brain stem. In 41 of these units we determined the amount of tension decrease during a partially fused tetanus (sag) and the ratio between peak tetanic force after 2 min of intermittent tetanic stimulation and initial tension (fatigue index). Muscle fibers of 24 motor units were identified by the glycogen depletion method and characterized in serial sections with monoclonal antibodies against type IID, IIA, ''cardiac'' alpha, and I isoforms of myosin heavy chain (MHC).2. The motor units had TCTs ranging from 13 to 32 ms. The majority of the units showed forces <35 mN. The TPFs were larger and varied more for motor units with short and intermediate TCTs than for units with long TCTs. There is a small but statistically significant negative correlation between the motor unit TPF and the TCT.3. All units exhibited ''sag'' and, with the exeption of one, had fatigue indexes > 0.75. The studied rabbit masseter motor units can therefore be classified as fast, fatigue-resistant, except for one that belonged to the FF (fast, fatigable) category. No slow units were represented in the sample pool. Significant correlations were not found either between TCT and the amount of sag or between TCT and the fatigue index.4. Immunohistochemical analysis showed that the EF unit had fibers containing only IID-MHC. Five other units were found with a single MHC-three with IIA-MHC and two with alpha-MHC. In three other units all fibers showed one combination of two MHCs (1 IIA/IID, 1 IIA/alpha, and 1 alpha/I). The remaining 15 units contained two MHCs spread unevenly over the constituting fibers. Large variations in myosin composition of fibers within one motor unit cast doubts on the presumed dominant neuronal influence on myosin expression in the adult animal.5. We found a close, statistically significant correlation between the TCT and the estimated MHC content of the units: the TCT was 13 ms for the IID unit, 18 ms for the pure IIA units, and 28 ms for the pure alpha units. Units with two MHCs had intermediate TCTs; units with alpha/I-MHC mixtures had TCTs of 29-30 ms. No pure MHC-I units were identified.6. We conclude that the almost homogeneous population of masseter motor units, as judged from classical physiological criteria, in reality shows a continuous spectrum of contraction speeds, accurately matching a continuous spectrum of MHC mixtures. Contraction speed of fibers decreases dependent on their MHC content in the order IID-IIA-alpha-I. Our results show that only combinations of neighboring MHCs on this list occur in fibers and motor units.